1. Field of the Invention
This invention relates to active matrix substrates and a method of manufacturing active matrix substrates.
2. Description of the Related Art
An active matrix display where an active element is provided for each pixel can realize a flat-type display with high picture quality. Of displays of this type, a liquid-crystal display (LCD) that uses liquid crystal as an optical shutter and drives each pixel with an active element, such as a TFT, has been widely used.
A known organic electroluminescence (EL) display is such that EL material capable of emitting red, green, and blue (RGB) light is formed by inkjet techniques and mask deposition techniques to make pixels, and each pixel is driven by an active element, such as a TFT. The organic EL display can also display a full color image on a thin panel screen.
In the prior art, to form the pixel section of such an active matrix flat-type display, each component layer is formed on a glass substrate using vacuum thin layer processes, such as CVD or sputtering techniques. The layers are then subjected to microscopic processing by dry etching or wet etching and by photolithography. These processes are repeated for each of the layers, including a semiconductor layer, an electrode layer made of metal or the like, and an insulating layer. As a result, the number of processes increases, resulting in an increase in the cost.
In the flat-type display, the active elements are formed not on the entire surface of the substrate but on a part of each pixel. Thus, it is a waste of time and labor to form all the active elements provided on the glass substrate. Such a wasteful forming method sets the economical limits to the formation of a large-area display.
In contrast, one known method is such that a plurality of active elements are formed very densely beforehand on an element formation substrate and that they are transferred to an intermediate substrate and further transferred to a display substrate (or a final substrate), and thereafter, passive structures, including interconnections and pixel electrodes, are formed, thereby reducing the cost (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2001-7340).
FIG. 1 shows part of the process of transferring active elements from an intermediate substrate to a final substrate in Jpn. Pat. Appln. KOKAI Publication No. 2001-7340. As shown in FIG. 1, on a final substrate 2501, patterned scanning lines 2503, interlayer insulators 2504, patterned signal lines 2502, a planarization layer 2505 are stacked one on top of another in this order. On the planarization layer 2505, an adhesion layer 2506 is provided. In the interlayer insulators 2504 and planarization layer 2505, contact holes for subsequent wiring are made in the areas corresponding to the signal lines 2502 and scanning lines 2503.
At the intermediate substrate 2507, TFTs 2510 are formed via a temporary adhesion layer 2508. The TFTs 2510 are covered with a protective layer 2509. At the bottom of each of the TFTs 2510, an under layer 2511 is provided to protect the TFT.
To transfer the TFTs 2510 from the intermediate substrate 2507 to the final substrate 2501, the TFTs 2510 to be transferred are aligned with the adhesion layer 2506. Then, light is projected through a shading mask 2512 which has an opening only in this area. Then, the adhesion of the temporary adhesion layer 2508 is weakened and the under layer 2511 is bonded to the adhesion layer 2506, thereby performing transfer.
In this method, TFTs 2510 requiring many manufacturing processes are formed on the element formation substrate with a high element density. The TFTs 2510 are transferred once from the element formation substrate to the intermediate substrate 2507. Then, the TFTs 2510 on the intermediate substrate 2507 are further transferred onto the final substrate 2501. At that time, use of a large element formation substrate and a large intermediate substrate 2507 enables TFTs 2510 arranged with a high density to be selected at regular intervals and transferred to the final substrate 2501 by way of the intermediate substrate. Using such a method makes it possible for TFTs 2510 used for many substrates to be formed on a single element formation substrate, which reduces the cost equivalent to the ratio of the TFT density of the element formation substrate to that of the final substrate.
When the TFTs 2510 are transferred to the final substrate 2501 by the method shown in FIG. 1, the TFTs 2510 are pressed against the adhesion layer 2506. In this case, it is difficult to control the force with which the TFTs 2510 are pressed. When the adhesion layer is pressed, the following problem arises: the height of the adhesion layer 2506 spread sideways differs from that of the adhesion layer 2506 not spread or the TFTs 2510 are not in parallel with the final substrate. Since the height of the TFTs 2510 provided on the final substrate 2501 and the angle of the TFTs 2510 with the final substrate 2510 are not controlled, the formation of interconnections after the TFTs 2510 are formed is difficult.
Furthermore, when the TFTs 2510 are pressed against the adhesion layer 2506 for transfer, there is a possibility that the adhesion layer 2506 will spread sideways and the adjacent TFTs 2510 also adhere to the adhesion layer 2506. Therefore, to prevent the adjacent TFTs 2510 from adhering to the adhesion layer 2506 even when the adhesion layer 2506 spreads sideways, the adjacent TFTs 2510 have to be spaced a suitable distance apart from each other. As a result, the number of TFTs 2510 formed on a single element formation substrate decreases, which causes the problem of increasing the cost.
Therefore, there has been a need to realize an active matrix substrate which is capable of controlling the height of active elements from the surface of the final substrate in transferring the active elements and that enables the active elements to be almost parallel with the final substrate even via an adhesion layer, and a method of manufacturing such active matrix substrates.